The present invention relates generally to chopper-stabilized amplifiers, and more particularly to chopper-stabilized amplifiers and methods which result in very low input leakage current during output overload conditions.
Referring to FIG. 1, a conventional chopper-stabilized INA (instrumentation amplifier) 1A includes an operational amplifier 3 having its (+) input coupled by conductor 5 to one output of an input chopper 2. The (−) input of amplifier 3 is coupled by conductor 7 to another output of input chopper circuit 2. One input of input chopper 2 is coupled by conductor 4 to receive input voltage VIN+. Another input of input chopper 2 is coupled by conductor 8 and a feedback resistor 9 of resistance R2 to the output 6 of amplifier 3. Various parasitic capacitances, designated Cp, are coupled to conductors 5 and 7, respectively. Similarly, INA 1A also includes an operational amplifier 11 having its (+) input coupled by conductor 15 to one output of input chopper 10. The (−) input of amplifier 11 is coupled by conductor 13 to another output of input chopper 10. One input of input chopper 10 is coupled by conductor 12 to receive input voltage VIN−. Another input of input chopper 10 is coupled by conductor 18 and a feedback resistor 16 of resistance R2 to the output 14 of amplifier 11. Note that input choppers 2 and 10 in FIG. 1A can be similar to input chopper 2 in subsequently described FIG. 1B. A resistor 17 of resistance R1 is coupled between conductors 8 and 18. Various parasitic capacitances Cp are coupled to conductors 15 and 13, respectively. Input choppers 2 and 10 are clocked by a chopper clock signal CHOP_CLK. The differential input voltage ΔVIN applied between input conductors 4 and 12 is equal to VIN+−VIN−.
Note that amplifiers 3 and 11 each include a conventional output chopper circuit, which may be similar to output chopper 27 in subsequently described FIG. 1B. Output conductor 6 of amplifier 3 conducts the instrumentation amplifier output voltage VOUT+, and output conductor 14 of amplifier 11 conducts the instrumentation amplifier output voltage VOUT−, so the differential input voltage ΔVOUT produced by chopper-stabilized INA 1A is equal to VOUT+−VOUT−. The output chopper circuitry 27 in each of amplifiers 3 and 11 is clocked by chopper clock signal CHOP_CLK.
FIG. 1B is a schematic diagram of a conventional chopper-stabilized amplifier as shown in FIG. 2A of commonly owned U.S. Pat. No. 7,292,095 entitled “Notch Filter for Ripple Reduction in Chopper Stabilized Amplifiers” issued Nov. 6, 2007 to Burt et al. Input chopper 2 in FIG. 1B includes a (+) input terminal which receives an input voltage VINPUT+ and which is connected to one terminal of each of switches 41A and 44A. Input chopper 2 also includes a (−) input terminal which receives an input voltage VINPUT− and which is connected to one terminal of each of switches 42A and 43A. Switches 43A and 44A are controlled by an internal chopping clock signal Φ and switches 41A and 42A are controlled by an internal chopping clock complement signal /Φ. Note that Φ in FIG. 1B can be the same as or be derived from CHOP_CLK in FIG. 1A.
The (+) input of a suitable amplifier, such as transconductance amplifier 3, is connected to a second terminal of each of switches 43A and 41A, and the (−) input of transconductance amplifier 3 is connected to a second terminal of each of switches 42A and 44A. The (+) output of transconductance amplifier 3 is coupled by conductor 56 to a first terminal of each of switches 42B and 43B of an output chopper circuit 27, and the (+) output of transconductance amplifier 3 is coupled by conductor 56 to a first terminal of each of switches 42B and 43B of output chopper circuit 27. An output conductor 58 of the chopper-stabilized amplifier shown in FIG. 1B is connected to a second terminal of each of switches 43B and 41B and conducts an output voltage VOUTPUT+, and another output conductor 59 of the chopper-stabilized amplifier is connected to a second terminal of each of switches 42B and 44B and conducts an output voltage VOUTPUT−. The switches in input chopper 2 and output chopper 27 may be implemented in various ways, for example by means of CMOS transmission gates. (Note that the operation of output chopper 27 does not significantly affect the input leakage current of the chopper-stabilized amplifier.)
The chopper-stabilized amplifier shown in FIG. 1B is essentially equivalent to the conventional chopper shown in FIG. 4 of the article “A Low Noise, Low Residual Offset, Chopped Amplifier for Mixed Level Applications” by M. Sanduleanu, A. van Tuigal, R. Wasasenaar, and H. Walling a, Electronics, Circuits and Systems, 1998 IEEE International Conference, September 7-10 in Lisboa, Portugal, ISBN 0-7803-5008-1, which is incorporated herein by reference. FIG. 5 of the Sanduleanu article shows another similar circuit in which output chopping switches are incorporated within rather than after the folded-cascode stage of the transconductance amplifier; the associated text of the Sanduleanu article describes the advantages of the implementation of FIG. 5 over the implementation of FIG. 4 for some applications.
For the normal operation of chopper-stabilized INA 1A in FIG. 1A, the following two equations are valid, assuming high amplifier gain:
                                          DV            OUT                    =                                    (                              1                +                                                      2                    ⁢                    R                    ⁢                                                                                  ⁢                    2                                                        R                    ⁢                                                                                  ⁢                    1                                                              )                        ⁢                          DV              IN                                      ,                            Equation        ⁢                                  ⁢        1            where ΔVOUT=VOUT+−VOUT− and ΔVIN=VIN+VIN−;ΔV=0.  Equation 2:
The allowed input dynamic range of chopper-stabilized INA 1A is dependent on the gain term in Equation 1A, which is determined by resistors R1 and R2. Under the normal operating conditions for which Equations 1 and 2 are valid, the input leakage current ILEAKAGE is very small, e.g., a few picoamperes. However, when ΔVIN exceeds the dynamic input voltage range limit of chopper-stabilized INA 1, its output becomes “saturated”. Then Equation 2 is no longer valid, and leakage current ILEAKAGE increases dramatically, according to the expressionILEAKAGE=2Cp×ΔV×fCHOP,  Eqn. 3:where fCHOP is the input chopping frequency. The value of ILEAKAGE given by Equation 3 may be in the micrompere range, and is too high to be acceptable in many applications. For example, there are medical safety specifications which do not permit more than a certain amount of current, e.g., 1 microampere, to flow into or out of the human body. The ANSI/AAMI EC11 (American National Standard/Association for the Advancement of Medical Instrumentation) standard for diagnostic electrocardiographic devices limits the current flow into or out of the human body to less than 100 nA.
For biopotential measurements, it is important to determine whether or not the electrical connection of the electrode to the input of a chopper-stabilized amplifier has become loosened. For this purpose, a very small current (of the order of tens of nanoamperes) is injected into an electrode attached to a human body. The idea is that for an open electrode the amplifier input will reach the supply voltage rail and this can be detected. The excessive leakage current in the chopper-stabilized amplifier during saturation interferes with this detection as explained below. If the electrode has become disconnected, the input of the amplifier starts moving toward a supply voltage rail. This may cause the output of the chopper-stabilized amplifier to saturate for any gain greater than unity. If during saturation ILEAKAGE increases to a value larger than the current injected into or out of the human body, it may prevent the input signal from moving sufficiently close to the supply voltage, thereby interfering with reliable detection of the disconnection of the electrode from the human body. Therefore, it is essential to keep the leakage current low even during output overload conditions.
During normal amplifier operation, a very small input current, typically less than a picoampere, is all that flows if chopper stabilization is not being utilized. However, if the amplifier is chopper-stabilized, ILEAKAGE may be as little as only a few picoamperes during normal operation, but if an amplifier overload condition (also referred to herein as a saturation condition) occurs, that value of ILEAKAGE may increase (e.g., by a factor of more than a million) to a number of micromperes.
The term amplifier “overload” or “saturation” as used herein refers to the condition wherein the product of the amplifier input voltage ΔVIN multiplied by the amplifier gain exceeds VDD−VSS. Most systems that include an amplifier try to correct a situation in which an amplifier output is attempting to exceed a supply voltage, i.e., where the amplifier output voltage is “saturating”. Often, this is accomplished by reducing the amplifier gain to unity to prevent or correct the saturation condition. A conventional technique for accomplishing this is to use gain control circuitry to reduce the amplifier gain to unity. Often, a DSP (digital signal processor) in the system first recognizes the saturation condition from digitized data representing the amplifier output voltage. The DSP then supplies control signals to the gain control circuitry to reduce the amplifier gain so as to eliminate the saturation condition.
However, users of currently available chopper-stabilized amplifiers see only a digitized representation of the output of the amplifier, and they see it only after a substantial delay. Consequently, the DSP observes the digitized output and then takes appropriate corrective action only after the substantial delay. Unfortunately, the foregoing technique for preventing amplifier overload/saturation conditions by reducing the amplifier gain is not acceptable as a way of reducing ILEAKAGE.
Thus, there is an unmet need for a chopper-stabilized amplifier having very low input leakage current during amplifier overload conditions.
There also is an unmet need for a chopper-stabilized amplifier which is capable of meeting very low input leakage current specifications required for any application which needs an amplifier having high input impedance, including applications for medical devices in which the maximum input leakage current is set forth by established standards.
There also is an unmet need for a chopper-stabilized amplifier which automatically limits increases of input leakage current that occur during amplifier overload/saturation conditions.
There also is an unmet need for a chopper-stabilized amplifier which avoids the need for a user to detect an amplifier overload/saturation condition and take corrective action to prevent excessive input leakage current.